This invention relates to an intelligent packet retransmission scheme, and in particular to an intelligent packet retransmission scheme that aims at improving the utilization of wireless links in environments where so-called fading phenomena prevail.
In wireless communication systems radio waves propagate through space as travelling electromagnetic EM waves. The energy of signals exists in the form of electrical E and magnetic H fields. Both electrical and magnetic fields vary sinusoidally with time. The two fields always exist together because a change in electrical field generates a magnetic field and a change in magnetic field generates an electrical field. Thus, there is a continuous flow of energy from one field to the other.
Radio waves arrive at a mobile station in a wireless communication system from different directions with different time delays. They combine via vector addition at the receiver antenna to give a resulting signal with a large or small amplitude depending upon whether the incoming waves combined to reinforce each other or cancel each other. As a result, a receiver at one location may experience a signal strength several tens of dB different from a similar receiver located only a short distance away. As the mobile station moves from one location to the other, the phase relationship between the various incoming waves also changes. Thus, there are substantial amplitude and phase fluctuations and the signal is subjected to fading. It should also be noted that whenever relative motion of the mobile station exists, there is also a Doppler shift in the received signal.
In the mobile radio case, the fading and Doppler shift occur as a result of motion of the receiver through a spatially varying field. Further, it also results from the motion of scatterers of the radio waves, e.g., cars, trucks, vegetation. Thus, the effect of multipath propagation is to produce a received signal with an amplitude that varies quite substantially with location. In addition, at UHF and higher frequencies, the motion of scatterers also causes fading to occur even if the mobile set or handset is not in motion.
FIG. 19 illustrates the overall fading characteristics of a mobile radio signal. Here, the rapid fluctuation caused by the local multipath is known as fast fading or Rayleigh fading.
FIG. 20 shows the basic mechanism underlying this fading phenomenon. As mobile telephony becomes more and more popular, the subscriber density in particular in cities is continuously increasing. Thus, using a mobile station in such an environment gives rise to the amplitude and phase fluctuation explained above. As shown in FIG. 20, radio waves arrive from different directions so that the signal takes more than one path from the transmitting antenna T to the receiving antenna R. The signal is not received directly from the transmitting antenna, but also from other directions where it has bounced, e.g., building B1 to B6. Overall, the signal(s) reach(es) the mobile station MS via several reflections against these buildings B1 to B6.
This means that the received signal is the sum of many identical signals which differ, e.g., only in phase and to some extent also in amplitude. This eventually means that the sum of the identical signal turns out to be very close to zero and that the signal strength also comes very close to zero, the worst case fading dip.
As shown in FIG. 21, another kind of fading results from shadowing effects, i.e. the use of the mobile station in an environment with obstacles. According to FIG. 21 there may exist hills H and buildings B between the transmitting antenna T and the receiving antenna R of the mobile station MS so that the received signal is decreased in strength.
The fading caused by shadowing effects is called log-normal fading since the logarithm of the signal strength takes the form of a normal distribution around some mean value. Typically, the distance between two minima or fading dips is some 10 to 20 meters. A fading effect being strongly related to the log-normal fading is the so-called rice fading. In particular, in systems that rely on a free line of sight between the sending antenna T and receiving antenna R this effect occurs, when the line of sight is disturbed. In this case the signal strength will decrease dramatically when the line of sight is blocked and the receiving antenna only receives signals being reflected.
Further, according to FIG. 19 the third phenomenon reducing the signal strength versus distance is the path loss which occurs when the received signal becomes weaker and weaker due to an increasing distance between the transmitting antenna T and the receiving antenna R. The higher the frequency, the higher the attenuation.
Finally, as shown in FIG. 22 the transmission of data packets leads to the phenomenon of time dispersion. Time dispersion, too, has its origin in reflections but contrary to the multipath fading reflected signals come from objects far away from the receiving antenna R, i.e. in the order of kilometers. Time dispersion leads to inter symbol interferences where consecutive symbols interfere with each other so that it is difficult on the receiver side to decide which actual symbol has been detected.
Since reflected signals come from objects far away instead of a single transmitted pulse there may be received a plurality of distinct pulses according to the long distances and associated delay times. Therefore in case, e.g., the sequence 1, 0 is sent from the transmitting antenna as shown in FIG. 22 in case reflected signals arrive exactly 1 bit time after the direct signal, the receiving antenna will detect a value of 1 from the reflected signal at the same time as it detects a value of 0 from the direct wave so that both symbols interfere.
As outlined above, all wireless systems have to cope with the unreliable nature of the radio link. The loss of single bits or a row of bits belongs to the natural character of a radio link. Also, the loss of information is caused by varying signal strength making communication impossible in case it drops under a certain threshold.
To overcome the problem of varying signal strength a number of mechanisms are used at different protocol levels. These mechanisms are, e.g., forward error correction, power control, frequency hopping and retransmission.
According to the invention, in particular the case of retransmission in case of data loss and the improvement of the underlying schemes are considered. Here redundancy may be added to the data to be transmitted which allows to detect transmission errors at the receiving side. The amount of redundancy is determined such that the detection of bit errors is enabled, but not the correction thereof. In case the receiver detects such a bit error, it requests the transmission of the responding data once again. This is usually achieved through a negative acknowledgement sent to the sender. Further, acknowledgement must be sent for every transmitted data item over the unreliable radio link. These acknowledgements can be put together and acknowledge either a number of data items or each data item separately.
The appropriate way of sending acknowledgement is decided according to the amount of additional signalling information and delay experienced by the end user that has to taken into account. Once the acknowledgement in retransmission scheme is specified every data loss is handled in the same way irrespective of the type of disturbance. Even worse, data not crossing a certain threshold are not treated at all since the complete connection is lost in case the duration of disturbance becomes too long.
In view of the above, the object of the invention is to achieve an improved utilization of a radio link channel accessed by multiple users in a wireless packet-oriented transmission system.
According to one aspect of the invention this object is achieved through a transmission apparatus for a wireless communication system.
One important advantage of the invention is that the invention provides a mechanism for improved utilization of wireless data links in environments which have to cope with fading phenomena by distinguishing different disturbances of the radio link. Thus, it is possible to achieve an impact of different fading effects on to a retransmission scheme as the retransmission scheme depends on the kind of disturbance and the reason therefor.
Another advantage of the invention is that it is not restricted to a specific wireless communication system but may be applied to any system wherein data is transmitted in data packets, example given the GPRS General Packet Radio Service System or the ATM wireless communication system.
Further, the benefits of the invention grow with the increase in transmission rate as here the use of radio link resources for other users in case of the detection of long lasting disturbances allows for the transfer of an increased amount of data.
Overall, according to the invention retransmission attempts are only made in case there is a chance for success.
Also, according to another aspect of the invention this object is achieved through a mobile unit for a wireless communication system according to claim 31.
Thus, the mobile unit according to the present invention is adapted to take into account that disturbances may occur on the radio channel link in the mobile unit to the respective transmission apparatus and that information on the kind of disturbance may easily be derived from the signal received at the mobile unit.
Therefore, different disturbances, i.e. Rayleigh fading, log-normal fading, path loss fading, etc., can be classified at the mobile unit. According to the present invention it is proposed to then retransmit the information on the kind of disturbance to the related transmission apparatus that therefore may avoid any unsuccessful data packet transmission attempts.
Since the acknowledgement message sent by the mobile unit already contains information classifying the kind of disturbance, it is possible to immediately reassign transmission channels within the transmission apparatus without carrying out additional transmission attempts. Also, using the mobile unit according to the invention it is possible to take precaution against different disturbance phenomena with an improved monitoring of these disturbance phenomena. Also, using location specific information on the disturbances it is possible to enhance the utilization of the scarce radio resources.